Evaluation of cobalt oxides for calcium battery cathode applications

Abstract The identification of potential cathode materials is a requirement for the development of a rechargeable calcium based battery technology. In this work, we use Density Functional Theory (DFT) calculations to explore the electrode characteristics of three ternary calcium cobalt oxides with distinct Co O dimensionality: 3D-Ca 2 Co 2 O 5 (brownmillerite type structure), 2D-Ca 3 Co 4 O 9 (a misfit compound) and 1D-Ca 3 Co 2 O 6 (K 4 CdCl 6 structural type). For the three compounds calculations predict Co 3+ /Co 4+ voltages in the 3–4 V range, with a volume variation below 8% upon Ca deinsertion. Further Co 4+ oxidation is predicted at too high voltages not reachable in practice. The maximum specific capacities are therefore 192 mAh/g (Ca 2 Co 2 O 5 ), 165 mAh/g (Ca 3 Co 4 O 9 ) and 160 mAh/g (Ca 3 Co 2 O 6 ). The potential application of Ca 2 Co 2 O 5 is discarded based on a large energy barrier for Ca diffusion (1.3 eV). With energy barriers for Ca diffusion of 0.9 eV, the 2D and 1D oxides are appealing as low rate cathode materials. To complete a previous experimental investigation, we have analyzed the reversibility of the Ca deinterclation reaction of 1D-Ca 3 Co 2 O 6 . It is found that a phase transformation takes place upon decalciation driven by the change in the electronic configuration of Co ions (from High Spin-trigonal prismatic Co 3+ to octahedral Low Spin-Co 4+ ) and involving the Ca diffusion pathways.